The present invention concerns an apparatus for effecting the sweep of a beam of light strictly plane and parallel to a given axis and, more particularly, such an apparatus comprising a sweeping mirror, mounted on an axis of rotation substantially perpendicular to the said given axis and the plane of which substantially contains the said axis of rotation, and an optical device, such as a source of light, arranged substantially in the plane containing the said given axis and perpendicular to the axis of rotation.
Such apparatuses are already well known in the prior art. Thus the French patent application filed on Feb. 4, 1987 under No. 87.01360 describes an apparatus involving the sweep of a scene observed by successive lines, by means of a laser beam reflected by a mirror pivoting about an axis under the effect of a galvanometer.
It frequently happens to be necessary, in the course of sweeping, for the beam of light to remain contained in one and the same plane parallel to a given axis.
This is the case, in particular, in the abovementioned patent application, where the laser beam forms a spot of light in a given location of the scene, and where the image of this spot is formed on a small bar of photosensitive elements. It is therefore important that the image of the said spot be perfectly centered on each element of the bar, and consequently that the sweep be perfectly plane and that the plane of sweep be parallel to the axis of the bar.
The desired precision for this apparatus is of the order of several seconds of an arc for the angle of elevation of the reflected beam.
Inversely, it may be desired to make use of an apparatus comprising a photosensitive receiver, and to receive on this receiver only the light coming from a certain plane parallel to a given axis, effecting the sweep of this plane by means of a swiveling mirror reflecting the light coming from the said plane to the photosensitive receiver.
Now, three sources of curvature of the plane of sweep are possible in an apparatus designed as described above.
First of all, the source of light or the photosensitive receiver may be located slightly above or below the plane containing the given axis perpendicular to the axis of rotation, i.e., the beam of light does not have a zero incidence with regard to the said plane.
In addition, sweeping mirrors are cemented to their axis of rotation so that, in theory, the plane of the mirror passes through this axis. In practice, it proves impossible to find mirrors respecting this condition with the desired precision which may attain an arc of several seconds: commercial galvanometric mirrors, as a matter of fact, exhibit defects of parallelism with their axis which may exceed an arc of 20 minutes.
Finally, the axis of rotation of the mirror itself may not be strictly perpendicular to the given axis.
It is understood that, if the aforementioned first and third defects may be limited by careful construction of the apparatus, the same does not apply to the second defect, which is inherent in the mirror itself.